This present invention relates to two-phase marine propulsion systems in general and more particularly to underwater two-phase ramjet engines.
Various attempts have been made to develop water breathing derivatives of gas breathing jet engines for significantly broadening the performance envelope of high speed marine vessels. Fundamentally, water breathing ramjet engines operate on the principle of energizing and accelerating water with compressed gas or the combustion products of a gas generator as described in U.S. Pat. No. 3,171,379 entitled "The Hydro-Pneumatic Ram-Jet" to Schell et al. and commonly known as the "Marjet". According to Newton's 1.sup.st Law, the propulsion system exerts thrust by applying an equal and opposite force upon an adjacent medium. In the case of a fluid medium, according to Newton's 2.sup.nd Law, the force is equal to the rate of change of the fluid's momentum. The part of the fluid which undergoes the momentum change is called the "working fluid". In an underwater two phase ramjet engine propulsion unit, the working fluid is a two-phase mixture of water and gas, preferably air. The bubbly flow is typified by high density with compressibility due to the liquid phase and the gaseous phase, respectively.
Although the Marjet is the most developed system of its kind described in the prior art, it nevertheless suffers from several significant disadvantages which can be attributed to its lack of commercialization. The disadvantages of the Marjet include: First, poor mixing efficiency leading to low total propulsion efficiency. Second, gas introduction through a homogeneous porous jacket creating bubbles with a very narrow size distribution, thereby limiting the maximum volumetric portion of gas in the two-phase working fluid and so significantly limiting the craft's agility. Third, the inability to convert the gas's thermal energy into thrust power. Fourth, poor acceleration capability near stagnation and at low speed and limited acceleration potential, yielding inability to dash over the drag hump of hydrofoils or hovercraft. And still other disadvantages include that the thrust level is coupled with cruise speed, the propulsion unit does not display thrust reversal or integral steering capability and that propulsion and other hydrodynamic functions such as: sea keeping, active stabilization, lift, steering and thrust reversal are each carried out by dedicated systems.
Other developments include the Hydro-Pulse-Jet as described in Los Alamos National Laboratory Report LA-10358-MS, May 1985 in which the pulse jet device was considered for the propulsion of torpedo missiles. The only advantage of this development is its high speed capability while its disadvantages include it being complex, unsafe, water pollutant, very heavy, inefficient, costly, etc.
Another development includes the Gas-Augmented-Water-Jet as described in Report N 00014-75-C-0936 for the Office of Naval Research, Auburn University Ala., Mech. Eng. Department, November 1976 in which a water pump with an additional gas booster unit is provided in the pump's exhaust duct. The gas booster is unable to operate without the waterjet pump prior to it and, therefore, this arrangement has all the disadvantages of an impeller-based waterjet, plus the extra complexity of the gas booster, in exchange for extra power at high speed cruise.
Yet another development includes the "Water-Augmented-Gas-Jet" as described in U.S. Pat. No. 3,808,804 to Scott-Scott in which a propulsion unit includes a gas breathing turbofan engine, incorporating a mist booster unit in the exhaust duct, fed through water injectors, pipe lines and water pumps. This arrangement appears promising for high speed applications, but has severe safety and efficiency limitations when maneuvering in a harbor, near other craft, and at low speed.
The object of the present invention is to provide a novel two-phase underwater ramjet engine, free of the above mentioned disadvantages.